1. Technical Field
The invention relates to power amplifiers. More particularly, the invention relates to a method and apparatus for linearizing a power amplifier.
2. Description of the Prior Art
The efficiency of an RF power amplifier has a significant impact on the battery life of a portable device, such as a portable transmitter, because the amplifier typically consumes the most amount of power used by the device. Efficient power amplifiers are therefore highly desirable for portable transmitters. Efficient class C, D, E, and F power amplifiers are only capable of generating constant-amplitude outputs. However, many recent transmitter designs require a non-constant amplitude RF output to maximize the data rate within a given channel bandwidth.
A suitable linear RF amplifier may be manufactured using gallium arsenide devices. However, gallium arsenide devices are presently considered too expensive for many applications. While MOS is the preferred process for manufacturing semiconductor devices, due to its low cost of fabrication and high yields, MOS has traditionally been unsuitable for fabricating linear RF amplifiers due to its lack of linearity when used to implement a high efficiency amplifier. Such poor linearity introduces a significant amount of distortion into the amplifier's output signal. Many different linearization schemes have been proposed in the art to achieve a linear and efficient power amplifier.
The design of traditional linear power amplifiers normally involves a trade-off between efficiency and linearity. Polar modulation is a technique known in the art that simultaneously achieves linearity and efficiency in an RF power amplifier. Polar modulation is also known as envelope elimination and restoration (EER). In this approach, an RF input signal is decomposed into its polar components, i.e., phase and magnitude. These two polar components are amplified independently and are then recombined to generate an amplified, linear RF output signal. The phase component of the RF input signal is typically amplified by a constant-amplitude amplifier that is optimized for efficiency. The magnitude or envelope component of the RF input signal is typically amplified by a switching-mode power supply that operates as the power supply for at least the output stage of the constant-amplitude amplifier.
Various approaches to the use of polar modulation have been described by L. Kahn, Single-Sided Transmission by Envelope Elimination and Restoration, Proc. IRE, July 1952, pp. 803-806; and by M. Koch, R. Fisher, a High-Frequency 835 MHz Linear Power Amplifier for Digital Cellular Telephony, 39th IEEE Vehicular Technology Conference, May 3, 1989.
FIG. 1 is a block schematic diagram of a traditional RF amplifier 10 that employs the above-described envelope elimination and restoration technique. In the amplifier shown in FIG. 1, an RF input signal 12 is first decomposed into its polar components. These polar components comprise phase, which is a constant-amplitude signal, and magnitude, which is a low-frequency envelope signal. The phase and magnitude components are amplified independently along separate paths 15 and 11, respectively. The phase and magnitude components are then recombined to generate the linearly-amplified RF output signal 19.
The phase component is extracted from the RF input signal by the limiter 16, and is amplified by an efficient constant-amplitude amplifier that may comprise the nonlinear preamplifier 17, and the efficient, non-linear phase output stage 18. The magnitude component, which has a bandwidth comparable with the channel bandwidth, is extracted from the RF input signal by the envelope detector 13, and is amplified by the linear baseband amplifier 14. To maximize efficiency, the linear baseband amplifier 14 is implemented using a switching-mode power supply having a class-D amplifier as its output stage.
Existing implementations of switching-mode power supplies use pulse width modulation. The output of such a power supply is a square wave whose mark/space ratio represents the magnitude component of the RF input signal. However, using pulse width modulation to amplify the magnitude component introduces intermodulation distortion into the RF output.
It is therefore desirable to provide a high efficiency RF amplifier that can be fabricated using a low-cost process, such as MOS, and that provides linear amplification of the RF input signal.